The present invention relates generally to AC power supplies and, more particularly, to an oscillator for an AC power supply capable of delivering variable frequency output signals having precisely programmable phase angles.
In a wide variety of applications, AC power, either single phase, two phase, three phase or any multiple phase, manifested in the form of a plurality of variable frequency power signals characterized by precisely controlled phase angles must be generated to enable operation of an electrical device or apparatus. For example, power supplies delivering 90.degree. quadrature outputs as well as 120.degree. phase shifted outputs are quite common, both types of outputs being characterized by phase angle displacements which are integer multiples of 30.degree.. In fact, most commonly used multiple phase power signals exhibit phase angle variations or displacements which are integer multiples of 30.degree..
Conventionally, variable frequency power supplies utilize oscillators which are of two general types; namely, analog and digital. Analog oscillators are generally characterized by a rather simple design and provide relatively stable power signals for most applications although certain disadvantages arise when such an oscillator is incorporated in a power system. In particular, in order to maintain frequency and phase relationships to the degree of accuracy required, precision analog components frequently having matched temperature tracking characteristics must be utilized in the critical circuits of the oscillator. Also, amplitude stabilization components are normally needed to compensate for the effects of frequency and temperature changes on the amplitude of the output signals. The use of such components significantly increases the cost of analog oscillators which, nevertheless, typically still exhibit some inaccuracies due to such factors as component and temperature variations.
Digital oscillators, on the other hand, utilize crystal references to maintain output signal accuracy. Since the phase accuracy is digitally controlled, it is not sensitive to frequency, amplitude or temperature variations and the output frequency response is only a function of any filtering in the signal conditioning circuits. Unfortunately, the filtering circuits are normally quite complex and do impose various limitations on system operation. In one known form of digital oscillator, a binary counter is used to control the output frequency by dividing a reference frequency by a selected dividing ratio, the divided frequency signal being then used to clock an address counter for addressing a suitably programmed memory at a corresponding rate. However, since the clock signal is derived by dividing the reference frequency, it is not always exact so that, in order to achieve high accuracy at all frequencies, the reference frequency must be uncommonly high. It is also known to construct a digital oscillator using an accumulator to address a memory device, see, for example, U.S. Pat. No. 3,735,269. In accordance with this technique, the number of addressing steps used to generate a waveform is changed in order to vary the frequency which results in a highly distorted waveform at high frequencies necessitating the use of a rather complex filter. Also, phase angle programming is not readily achievable using this approach.
Another known type of digital oscillator employs a phase locked loop to generate the frequency for clocking the memory controlling address counter. This approach has the disadvantage of requiring a certain finite lock time and requires an excessive number of components to implement the phase locked loop. Also, as in the case of other digital oscillators embodying memory devices, the required memory capacity, especially when such must be duplicated for multiple phase outputs, often imposes severe limitations on system design.